Re: [PATCH 2/2] hugepage: Allow parallelization of the hugepage fault path

[Eric B Munson <emunson@mgebm.net>]

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On Fri, 15 Jul 2011, Anton Blanchard wrote:

> From: David Gibson <dwg@au1.ibm.com>
> 
> At present, the page fault path for hugepages is serialized by a
> single mutex.  This is used to avoid spurious out-of-memory conditions
> when the hugepage pool is fully utilized (two processes or threads can
> race to instantiate the same mapping with the last hugepage from the
> pool, the race loser returning VM_FAULT_OOM).  This problem is
> specific to hugepages, because it is normal to want to use every
> single hugepage in the system - with normal pages we simply assume
> there will always be a few spare pages which can be used temporarily
> until the race is resolved.
> 
> Unfortunately this serialization also means that clearing of hugepages
> cannot be parallelized across multiple CPUs, which can lead to very
> long process startup times when using large numbers of hugepages.
> 
> This patch improves the situation by replacing the single mutex with a
> table of mutexes, selected based on a hash of the address_space and
> file offset being faulted (or mm and virtual address for MAP_PRIVATE
> mappings).
> 
> From: Anton Blanchard <anton@samba.org>
> 
> Forward ported and made a few changes:
> 
> - Use the Jenkins hash to scatter the hash, better than using just the
>   low bits.
> 
> - Always round num_fault_mutexes to a power of two to avoid an
>   expensive modulus in the hash calculation.
> 
> I also tested this patch on a large POWER7 box using a simple parallel
> fault testcase:
> 
> http://ozlabs.org/~anton/junkcode/parallel_fault.c
> 
> Command line options:
> 
> parallel_fault <nr_threads> <size in kB> <skip in kB>
> 
> 
> First the time taken to fault 128GB of 16MB hugepages:
> 
> # time hugectl --heap ./parallel_fault 1 134217728 16384
> 40.68 seconds
> 
> Now the same test with 64 concurrent threads:
> # time hugectl --heap ./parallel_fault 64 134217728 16384
> 39.34 seconds
> 
> Hardly any speedup. Finally the 64 concurrent threads test with
> this patch applied:
> # time hugectl --heap ./parallel_fault 64 134217728 16384
> 0.85 seconds
> 
> We go from 40.68 seconds to 0.85 seconds, an improvement of 47.9x
> 
> This was tested with the libhugetlbfs test suite, and the PASS/FAIL
> count was the same before and after this patch.
> 
> 
> Signed-off-by: David Gibson <dwg@au1.ibm.com>
> Signed-off-by: Anton Blanchard <anton@samba.org>

Tested-by: Eric B Munson <emunson@mgebm.net>

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